A primary focus and application of the present invention is in the field of radio frequency (RF) amplifiers for transmitters and/or receivers capable of use in wireless telecommunication units. The third generation partnership project (3GPP™) is a mobile (wireless) communications collaboration between groups of telecommunications standards associations. One of the currently developed standards is the long term evolved (LTE™) standard. LTE™ is a standard for high-speed wireless communication for mobile devices and data terminals, based on the global system for mobile (GSM™) communications or Enhanced Data GSM Environment (EDGE) and universal mobile telecommunications standards (UMTS)/High Speed packet access (HSPA) technologies. These technologies increase the capacity and speed using different radio interfaces together with providing core network improvements.
Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes and mechanisms that can better utilise limited available communications bandwidths. Carrier aggregation is used in LTE-Advanced in order to increase the bandwidth, and thereby increase the communication bit-rate, where a maximum of five component carriers (of up to 20 MHz each) can be aggregated, hence providing a maximum aggregated bandwidth of 100 MHz.
The easiest way to arrange aggregation is to use contiguous component carriers within the same operating frequency band (as defined for LTE™), so called intra-band contiguous carrier aggregation (ICA).
For non-contiguous carrier aggregation (NCCA) cases, the bands are separated by one, or more, frequency gap(s). Inter-band non-contiguous carrier aggregation is a form of carrier aggregation that uses different frequency bands. It is particularly useful where there is fragmentation of frequency bands, some of which may be, say, only 10 MHz wide. For a subscriber unit (sometimes referred to as a mobile station or a user equipment (UE) in GSM or LTE™ terminology), a use of multiple transceivers within the single device is often required, with the usual impact on cost, performance and power. In addition to this there are also additional design complexities resulting from the need to reduce intermodulation and cross modulation from multiple (e.g. two) transceivers.
In particular, in radio frequency designs that support wide operational bandwidth, or that support carrier aggregation across multiple supported carrier frequencies, multiple low noise amplifiers (LNAs) and/or programmable-gain amplifier (PGAs) may be used in parallel. As these LNAs or PGAs operate at high or very high radio frequencies, the signal paths from multiple amplifiers are typically combined using inductors or transformers.
LNAs and PGAs are found in radio communication systems, as well as medical instruments and electronic equipment. A typical LNA or PGA may supply a power gain of 100 (i.e. 20 decibels (dB)), whilst decreasing a signal-to-noise ratio by less than a factor of two (i.e. exhibiting a 3 dB noise figure (NF)). Although LNAs are primarily concerned with weak signals that are just above the noise floor, they must also be designed to consider the presence of larger signals that may cause intermodulation distortion.
Referring to FIG. 1, a known radio frequency (RF) amplifier circuit 100 is illustrated, where multiple RF amplifiers 130, 140 are connected in parallel. The outputs of each of the respective multiple RF amplifiers 130, 140 exhibits a parasitic capacitance 135, 145. The exhibited parasitic capacitances can result from overlapped area between gate and source/drain ports, as well as result from the diode (effect) between the ports.
The output of each of the respective multiple RF amplifiers 130, 140 is connected to a shared inductor (L1) 115. In some instances, the shared inductor (L1) 115 may be a transformer input inductance of a RF transformer 110. The RF transformer is provided by a voltage supply 105 and includes a transformer output inductance 112.
In some applications, for example lower frequency applications of, say, less than 1 GHz, the parasitic capacitances 135, 145 have little or no effect on the performance of the RF amplifier circuit 100. Thus, this illustrated architecture is the simplest way for inductor or transformer sharing of multiple RF amplifiers 130, 140 that are connected in parallel. However, in some instances, for example at frequencies higher than 1 GHz, the respective exhibited parasitic capacitances 135, 145 have an increasingly adverse frequency-response or non-linearity effect (as they emanate from cascade devices, which are nonlinear compared to a metal-oxide-metal (MOM) capacitor), when they are connected to inductors or transformers that combine the outputs of the respective RF amplifiers 130, 140. In particular, when a large number of RF amplifiers are connected in parallel, the inventors of the present invention appreciated that when the second RF amplifier 140 is turned ‘on’, the path connecting the RF amplifier 140 to a first inductor 115 (or an input side of an RF transformer) is also connected to both parasitic capacitances 135, 145. Hence, the total amount of parasitic capacitance, and the effect thereon of the additional parasitic capacitance on other connected RF amplifiers, increases with the number of parallel RF amplifiers.
In some known applications, it is possible to reduce the effect of parasitic capacitances 135, 145 by using smaller cascade devices; however, such an approach causes a poorer linearity performance.
Thus, the inventors of the present invention have identified a need for a reduced loading effect due to such parasitic capacitances of parallel paths of RF amplifiers that are connected to such shared inductors or transformers. In particular, a more efficient solution is needed in order to support multiple parallel RF amplifiers, for example for supporting Inter-band Carrier Aggregation (ICA) and Non-Contiguous Carrier Aggregation (NCCA) applications in an LTE™ or similar system.